The spectral-domain variant of optical coherence tomography (“OCT”), called spectral-domain optical coherence tomography (SD-OCT), is a technique is a technology that is suitable for ultrahigh-resolution ophthalmic imaging. This technique has been described in Cense, B. et al., “Ultrahigh-resolution high-speed retinal imaging using spectral-domain optical coherence tomography”, Optics Express, 2004 and in International Patent Publication No. WO 03/062802. In addition, U.S. patent application Ser. No. 10/272,171 filed on Oct. 16, 2002 also relates to this subject matter. The axial resolution of an OCT system may be defined in terms of the coherence length (Lcoh), which can be determined by the center wavelength and bandwidth of the source and the index of refraction of the medium, as described in greater detail in Swanson, E. A. et al., “High-Speed Optical Coherence Domain Reflectometry”, Optics Letters, 1992, 17(2), pp. 151-153. The axial resolution of the OCT system can be improved by using an ultra broadband source, as provided in further detail in Drexler, W. et al., “Enhanced Visualization of Macular Pathology with the Use of Ultrahigh-Resolution Optical Coherence Tomography”, Archives of Ophthalmology, 2003, 121(5), pp. 695-706.
One potential difficulty that may arises from using ultra-broadband sources in a fiber-based OCT setup in, e.g., ophthalmic imaging is the presence of a chromatic dispersion in optically-dense materials like glass, tissue and water. Chromatic dispersion can lead to smearing of the coherence function and/or point spread function in the axial direction, which can significantly affect the image quality. Considerable amounts of dispersion can be tolerated if the dispersion in the two arms of the interferometer is balanced, thus creating a coherence function that would likely to be free from dispersion artifacts. However, when sample and reference arms contain different lengths of optical fiber or other dispersive media, a dispersion mismatch can occur. For example, in the sample arm of an OCT system, the analysis of an eye as a sample with unknown axial length may introduce an unknown amount of chromatic dispersion. Thus, the coherence function may be broadened by an unbalanced dispersion, and the peak intensity of the coherence function can decrease as well. A second order or a group-velocity dispersion can be compensated for using hardware by, e.g., changing the lens to grating distance in a rapid scanning optical delay line. The above has been described in detail in Tearney, G. J. et al., “High-Speed Phase- and Group-Delay Scanning with a Grating-Based Phase Control Delay Line”, Optics Letters, 1997, 22(23), pp. 1811-1813. However, this technique generally does not compensate for higher orders of dispersion. Alternatively, it is possible to balance a dispersion in the OCT system by inserting variable-thickness optical materials with different dispersion properties (such as BK7 and fused silica prisms) in the path of the reference arm or the sample arm. The number of materials with different optical properties that are inserted in the path of the reference arm or the sample arm may determine the number of orders of dispersion one can compensate. The axial length of an eye can vary from one person to another, thus changing the amount of dispersion between patients. Therefore a flexible technique for a dispersion compensation is desirable.
It is possible that, instead of using hardware for such compensation, to use software, and thereby a more flexible compensation easy to adapt to different eyes. Another publication describes a technique to provide an induced dispersion in the delay line of a time domain OCT system that equipped with an optical amplifier based source (e.g., AFC technologies, λ0=1310 nm, Δλ=75 nm) and compensated for dispersion artifacts in structural intensity images obtained of an onion. See de Boer, J. F. et al., “Stable Carrier Generation and Phase-Resolved Digital Data Processing in Optical Coherence Tomography”, Applied Optics, 2001, 40(31), pp. 5787-5790. Another publication describes a dispersion compensation which is induced by a glass sample. See Fercher, A. F. et al., “Dispersion Compensation For Optical Coherence Tomography Depth-Scan Signals By A Numerical Technique”, Optics Communications, 2002, 204(1-6), pp. 67-74. Their broadband spectrum is generated using a high-pressure mercury lamp. Other dispersion compensation techniques are described in Marks, D. L. et al., “Autofocus Algorithm for Dispersion Correction in Optical Coherence Tomography”, Applied Optics, 2003. 42(16), pp. 3038-3046, Marks, D. L. et al., “Digital Algorithm for Dispersion Correction in Optical Coherence Tomography for Homogeneous and Stratified Media”, Applied Optics, 2003, 42(2), pp. 204-217, and U.S. Pat. No. 5,994,690 which describe an algorithm that used an autocorrelation function to correct image data. However, the above-described problems have not been addressed adequately. Accordingly, there is a need to overcome such deficiencies.